2 research outputs found
Experimental and Theoretical Investigation of Homogeneous Gaseous Reaction of CO<sub>2</sub>(g) + <i>n</i>H<sub>2</sub>O(g) + <i>n</i>NH<sub>3</sub>(g) → Products (<i>n</i> = 1, 2)
Decreasing CO<sub>2</sub> emissions into the atmosphere
is key for reducing global warming. To facilitate the CO<sub>2</sub> emission reduction efforts, our laboratory conducted experimental
and theoretical investigations of the homogeneous gaseous reaction
of CO<sub>2</sub>(g) + <i>n</i>H<sub>2</sub>OÂ(g) + <i>n</i>NH<sub>3</sub>(g) → (NH<sub>4</sub>)ÂHCO<sub>3</sub>(s)/(NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>(s) (<i>n</i> = 1 and 2) using Fourier transform infrared attenuated total reflectance
(FTIR-ATR) spectroscopy and ab initio molecular orbital theory. Our
FTIR-ATR experimental results indicate that (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>(s) and (NH<sub>4</sub>)ÂHCO<sub>3</sub>(s) are formed
as aerosol particulate matter when carbon dioxide reacts with ammonia
and water in the gaseous phase at room temperature. Ab initio study
of this chemical system suggested that the reaction may proceed through
formation of NH<sub>3</sub>·H<sub>2</sub>OÂ(g), NH<sub>3</sub>·CO<sub>2</sub>(g), and CO<sub>2</sub>·H<sub>2</sub>OÂ(g)
complexes. Subsequent complexes, NH<sub>3</sub>·H<sub>2</sub>O·CO<sub>2</sub> and (NH<sub>3</sub>)<sub>2</sub>·H<sub>2</sub>O·CO<sub>2</sub>, can be formed by adding gaseous reactants
to the NH<sub>3</sub>·H<sub>2</sub>OÂ(g), NH<sub>3</sub>·CO<sub>2</sub>(g), and CO<sub>2</sub>·H<sub>2</sub>OÂ(g) complexes,
respectively. The NH<sub>3</sub>·H<sub>2</sub>O·CO<sub>2</sub> and (NH<sub>3</sub>)<sub>2</sub>·H<sub>2</sub>O·CO<sub>2</sub> complexes can then be rearranged to produce (NH<sub>4</sub>)ÂHCO<sub>3</sub> and (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> as
final products via a transition state, and the NH<sub>3</sub> molecule
acts as a medium accepting and donating hydrogen atoms in the rearrangement
process. Our computational results also reveal that the presence of
an additional water molecule can reduce the activation energy of the
rearrangement process. The high activation energy predicted in the
present work suggests that the reaction is kinetically not favored,
and our experimental observation of (NH<sub>4</sub>)ÂHCO<sub>3</sub>(s) and (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>(s) may be attributed
to the high concentrations of reactants increasing the reaction rate
of the title reactions in the reactor
Kinetic and Dynamic Investigations of OH Reaction with Styrene
The kinetics of hydroxyl radical
reaction with styrene has been
studied at 240–340 K and a total pressure of 1–3 Torr
using the relative rate/discharge flow/mass spectrometry technique.
In addition, the dynamics of the reaction was also studied using the
ab initio molecular orbital method. The reaction was found to be essentially
pressure independent over 1–3 Torr at both 298 and 340 K. At
298 K, the average rate constant was determined, using four different
reference compounds, to be <i>k</i><sub>styrene+OH</sub> = (5.80 ± 0.49) × 10<sup>–11</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. At 240–340
K, the rate constant of this reaction was found to be negatively dependent
on temperature with an Arrhenius expression determined to be <i>k</i><sub>styrene+OH</sub> = (1.02 ± 0.10) × 10<sup>–11</sup> exp[(532 ± 28)/<i>T</i>] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. Observation
of mass spectral evidence of adduct products and their respective
fragment ions suggests that the reaction proceeds with addition of
the OH to the vinyl carbons of the styrene molecule. Ab initio calculations
of both the addition and the abstraction pathways predict that the
addition pathways are more energetically favorable because of large
exothermicity and essentially barrierless transition state associated
with the additions, which is consistent with the experimental observations.
Using the styrene + OH rate constant determined at 277 K in the present
work, the atmospheric lifetime of styrene was estimated to be 4.9
h